Positron emission tomography (PET) is a powerful imaging modality that has had a major impact in oncology due to its ability in detecting disease, staging, assessing response to therapy, and identifying recurrent disease (1). In clinical whole-body PET imaging 18F-FDG is the most commonly used tracer for oncological studies where the primary task is the detection and quantification of lesions anywhere in the patient body. Although 18F-FDG is by far the most widely used tracer, new tracers are being developed for cancer diagnosis, detection of hypoxia, and angiogenesis. These applications promise to expand the role of PET even further in patient management and health care.
Breast cancer is the most prevalent form of cancer in women, with an incidence rate that is double that of the next higher form (lung cancer). The American Cancer Society (ACS) (2) estimates that that there will be 178,480 new cases of invasive breast cancer in women in USA in 2007. This represents 26% of all new cancer cases in women, with an expected mortality rate of 22%. In addition, the ACS also estimates the occurrence of another 62,030 new cases of the in situ type within the same year, about 85% of which will be of the ductal carcinoma in situ (DCIS) type. Several studies have shown that detection and treatment of breast cancer in the early stages leads to a decrease in breast cancer mortality rates (3-6). As a result, mammographic imaging with an average sensitivity rate of 80-90% is used as a screening tool for early detection of breast cancer. However, a recent study (7) of a large sample of patients has shown that the specificity of mammography is only 35.8% and results in a large fraction of false positive cases. PET imaging with its functional imaging capability can potentially play a complementary role in these situations.
Due to the general nature of routine clinical imaging, clinical whole-body PET scanners are designed to achieve reasonably good spatial resolution in the range of about 5-6-mm (8, 9) with large scanner ring diameters of about 90-cm. Breast imaging, on the other hand, is concerned with detecting, characterizing the nature, and monitoring the response of small tumors in the early pT1 (lesion size is as small as 5-mm or less (pT1a stage) (10) and pT2 (lesion size<2-cm) stages. In addition, due to the early stages of cancer onset, glucose and subsequent 18F-FDG uptake may also be low in these stages (10-13). Hence, a scanner with high spatial resolution is needed for accurately detecting the small lesions, while high scanner sensitivity provides accurate, quantitative images for short scan times. Poor spatial resolution and limited scanner sensitivity of clinical whole-body PET scanners, therefore, represent the most significant limitations in the use of PET as an important diagnostic application in breast imaging, since the ability to detect and quantify tumors<10-mm in size is greatly compromised (10, 14, 15). In fact, Avril, et. al. (10) have shown that the clinical detection sensitivity is <48% for all pT1 stage tumors and <13% for tumors <1-cm in size (pT1a and pT1b stages). The limited ability of clinical PET to detect and quantify the small, early stage, tumors therefore prevents its use for screening women for breast cancer (16).
As a result, whole-body PET is currently used primarily in the staging of breast cancer patients and determining the efficacy of treatment in these patients. However, the limited spatial resolution and sensitivity of these PET scanners prevents their use in characterizing and monitoring response of early stage tumors (stages I and II with lesion sizes<2 cm). Likewise, inadequate spatial resolution, limited scanner sensitivity, and the geometrical restrictions associated with whole-body scanners present early detection, efficacy, cost, and other practicality issues with respect to other localized sites of interest, for example, in the brain, prostate, or heart.
The cost of a small, high performance dedicated (e.g., breast, brain, prostate, cardiac) scanner would be significantly less than a clinical whole-body PET/CT due to the use of less detector material. In addition, the short scan times will also reduce the cost of an imaging study. A dedicated scanner has the advantage of reduced attenuation of coincident photons because they do not travel through intervening anatomical structures (such as the chest, in the case of breast imaging, which effects scanning sensitivity by factor of 10), and increased geometric efficiency due to a smaller ring diameter (about a factor of 4 for typical geometries).